1. Field of the Invention
The present invention relates to a semiconductor device having, for example, a Micro-Electro-Mechanical Systems (MEMS) element and its manufacturing method.
2. Description of the Related Art
Recently, the MEMS element having a minute movable unit formed by using a semiconductor manufacturing process on a silicon substrate is developed. This MEMS element is applied to, for example, a variable capacitor, an inductor, a switch, a sensor, a gyroscope, a mirror device, etc. As an actuator for constituting the MEMS element, there are, for example, an actuator using a static electricity, and an actuator using a piezoelectric film. A variable capacitor or a relay using these actuators have been developed (refer to, for example, U.S Pat. No. 6,377,438, U.S Pat. No. 6,359,374).
In the variable capacitor using the actuator including, for example, a piezoelectric film, an actuator layer is provided on a lower electrode. An upper electrode is provided on this actuator layer. It is ideal if this upper electrode and the lower electrode are parallel to each other. Further, it is preferable that the upper electrode and the lower electrode are disposed separately at a predetermined interval. However, as will be described below, it is difficult to constitute a variable capacitor of such ideal shape.
The variable capacitor is manufactured as follows. First, an insulating film is deposited on a substrate. A first electrode is formed as the lower electrode on this insulating film. Then, an interlayer insulating film is formed and an opening for exposing the first electrode is formed in this interlayer insulating film. Then, a filling material to become a sacrificial layer is deposited on the whole surface. Then, this filling material is planarized, and the opening is filled with the filling material. As a planarizing method, reactive ion etching (RIE), or a chemical mechanical polishing (CMP) is used. To assure the uniformity of the film thickness, the CMP is preferable. Thereafter, the actuator layer including a piezoelectric film is formed on the interlayer insulating film including the sacrificial layer. Subsequently, a second electrode is formed as the upper electrode on a portion corresponding to the sacrificial layer of the actuator layer. Finally, the sacrificial layer is removed.
However, when the sacrificial layer is planarized by the CMP, a phenomenon called dishing occurs if the opening is a wide pattern. Therefore, the front surface of the sacrificial layer filled in the opening is overetched. Thus, the front surface of the sacrificial layer becomes recessed. When the sacrificial layer is finally etched, the actuator layer is suspended in the opening. The second electrode and the first electrode cannot be arranged in parallel with each other. The mutual interval between the first and second electrodes is narrowed as compared with a designed value. Thus, it becomes difficult to acquire a required capacity. Further, there is another problem that the movable range of the second electrode is reduced.
Furthermore, when dishing arises on the front surface of the sacrificial layer, there is a possibility of a problem arising even in the case of etching the sacrificial layer. That is, when the sacrificial layer is etched, wet etching is frequently used. The actuator layer is suspended in the opening and the interval between the first electrode and the second electrode is narrowed. Then, a phenomenon called sticking due to a surface tension arises at the end of the wet etching. There is another problem that the actuator layer is adhered to the first electrode.